Planar structures comprising waveguide arrays such as multiple arm Mach Zehnder interferometers and simple two-arm interferometers have been used in various devices for processing optical signals. Waveguide arrays in such devices may for example be constructed using silica on silicon technology in which waveguides are defined by doped silica regions which have a higher refractive index than a surrounding cladding layer of germanium doped silica.
Arrays with large numbers of waveguides of different length formed using such structures have been proposed to provide filtering of optical signals, as for example disclosed by Dragone, IEEE Photon Technology Letters, September 1991, pp 812-815.
The inventor of the present invention has also disclosed in a co-pending application entitled "Optical Equalizer", the contents of which are incorporated herein by reference, an adaptive optical equalizer using an array of controlled waveguides to provide a transfer function which is synthesized by a Fourier analysis technique in order to achieve a desired characteristic of equalization between components in a wavelength division multiplexed optical signal.
The feasibility of such a waveguide array device to achieve adaptive equalization presents difficulties of manufacture using silica on silicon technology since, for example, reliance upon total internal reflection as the method of guidance means that the bend radius of waveguides must be sufficiently large to avoid excessive losses whereas the overall size of the planar structure is limited. A further difficulty is that of providing adequate means of controlling transmission parameters within waveguides. It has been proposed to use heating strips to locally vary the temperature of the waveguide material, thereby providing controlled adjustment of optical path length. The response time of such temperature control may not be adequate in some applications. There remains a need to provide an improved waveguide structure in such devices to facilitate manufacture and improve control.
A particular area of interest is the control of optical power levels in optical communications system to obtain optimum performance, the power level being required to be sufficient to establish a signal to noise ratio which will provide an acceptable bit error rate but without the power level exceeding a level at which limiting factors such as the onset of non-linear effects result in degradation of the signal. In wavelength division multiplexed transmission it is desirable to maintain each of the power levels of the individual wavelengths components at substantially the same level.
The inventor has disclosed in U.S. Pat. No. 5,513,029 a method of monitoring component power levels in WDM transmission using orthogonal low frequency dither signals and controlling component signal power to maintain optical performance.
It is also known from GB 2314714A that an imbalance of component signal powers in a WDM transmission is likely to occur at an optical amplifier stage, as used to boost signal power at stages in a long distance transmission utilising optical amplifiers such as erbium doped fibre amplifiers. Such amplifiers have a non-uniform gain characteristic as a function of wavelength which is variable in dependence upon the amplifier gain, this change in gain characteristic consequent on change of gain being commonly referred to as dynamic gain tilt.
There is therefore a need to provide optical filtering which is adaptive and which can be used in conjunction with optical amplifiers in order to maintain a preferred spectral profile of an optical signal.
It is known from J. D. Joannopoulous et al, Photonic Crystals: Moulding the Flow of Light, published 1995 by Princeton University Press, in Chapter 5, to provide two-dimensional photonic crystals within a planar slab of dielectric medium which exhibits a photonic band gap whereby the medium is non-transmissive to an optical signal within a defined frequency range for directions of propagation co-planar with the slab of dielectric medium. The slab is sandwiched between parallel slabs of material having lower refractive index to contain the optical signal by internal reflection. The photonic crystal is formed by providing in the planar slab of dielectric medium a lattice formed by lattice sites at which the dielectric properties of the medium are varied relative to the bulk properties of the dielectric medium such that a latticed region is formed which is essentially opaque to the optical signal and a waveguide region may then be formed by discontinuities in the periodic lattice, for example by omitting a contiguous set of lattice sites. The optical signal is therefore constrained to propagate through the waveguide region.
An advantage of such structures is that the waveguides may have a very small turn radius of the order of several wavelengths of the optical signal which compares favourably with a typical turn radius of the order of several centimeters which would be required for traditional core-cladding waveguides describes above which rely upon total internal reflection.
U.S. Pat. No. 5,651,818, Milstein et al, discusses in the introduction thereby a number of available techniques of manufacturing photonic band gap materials.
U.S. Pat. No. 5,784,400, Joannopoulous et al, proposes to utilise two-dimensional photonic band gap materials in an optical device in the form of a resonant cavity.
It is known from U.S. Pat. No. 5,389,943, Brommer et al, to utilise the frequency selective transmission properties of such two-dimensional photonic band gap materials in a filter in which transmitted light is modified in frequency response by the optical transmission characteristics of the bulk properties of the material. Further disclosed is the active control of material forming the lattice sites, such as by the application of an external field, in order to modify the refractive index of material at the sites and thereby actively control the transmissive properties of the filter.